The present invention relates to a process for the stabilization of the period of opening of electromagnetic injectors in a fuel injection system, particularly for engines of motor vehicles, such that the quantity of fuel injected is, for a given injector opening period, independent of the vehicle battery voltage. The invention also relates to a device for carrying out this process.
This invention relates in particular to a fuel injector of the electromagnetic type having a spring-loaded obturator piston movable by means of an operating solenoid, energization of which is controlled by electrical command signals of variable duration which keep the injector open for the required open time. For given conditions in the electrical circuit controlling the operating solenoid and the hydraulic fuel feeding circuit associated with the injector, the quantity of fuel injected at each injector stroke is directly proportional to the open time of the injector.
It is known that from the electrical standpoint the operating solenoid of an electromagnetic injector is equivalent to an inductance and a resistance in series. Upon energization of the solenoid the energizing current in the solenoid rises exponentially with a certain time constant, the instantaneous value of the current being directly proportional to the supply voltage, that is, the vehicle battery voltage.
The magnetic force opposing the preloading of the spring and acting on the obturator piston of the injector upon energization of the operating solenoid increases with the exponential growth of the energizing current and consequently movement of the piston commences only after a certain period of time following the moment of energization of the operating solenoid. Furthermore, upon de-energization of the operating solenoid there is a delay in the closing of the injector due to phenomena of magnetic hystersis and to the delay in the decay of the magnetic flux in the operating solenoid after switch-off of the energizing current.
It is known from the copending U.S. Pat. Application No. 736,684 that it is possible to compensate this lag between the command signals for opening and closure of an injector and the actual times of opening and closure of the injector if it is assumed that the moment of actual opening is the intersection of an experimentally predetermined threshold with the characteristic growth curve of the energizing current of the injector operating solenoid. According to that method, a counter which controls the duration on an injector control pulse is permitted to run during the battery voltage dependent lag time between the end of each injection control pulse and the time at which the injector actually closes. The counter begins running at the actual opening of the injector in the next injection cycle so that the battery voltage dependent lag time is always subtracted from the desired duration of the injection control pulse. This will result in an accurate elimination of both the battery voltage dependent decay time and the battery voltage dependent rise time of the injector energizing current only if the actual opening and closing times of the solenoid can be accurately determined.
The method described in the said U.S. Patent Application No. 736,684, which derives information from the waveform of the energizing current and, therefore, from the force applied to the injector obturator piston, presupposes that, for a given force on the piston there is always a corresponding mechanical movement which is always the same for an individual injector or for the various injectors in an injection system provided on a motor vehicle. Thus, it can always be assumed that an injector will open and close at experimentally determined values of the solenoid energizing current. This assumption requires a very accurate calibration of the injectors both with regard to their piston stroke and their spring preloading so as to ensure that, for a given energization current, the through-flow of the various injectors is uniform and such as to ensure constancy and repeatability in the movement of the injector obturator pistons as a function of time.